This invention relates to a perpendicular magnetic recording medium in which a ferromagnetic alloy layer is formed on a polymer film substrate, and to a method for preparing the same. More specifically, it relates to a perpendicular magnetic recording medium having prolonged durability and heightened reliability and a method for preparing the same.
In recent years, much attention has been paid to a perpendicular magnetic recording system for carrying out high-density recording by the utilization of remnant magnetization in a direction perpendicular to the surface of a magnetic recording medium.
As a typical example of the perpendicular magnetic recording medium for perpendicular magnetic recording, there is known a medium which permits forming a Co-Cr system alloy layer having an axis of easy magnetization in the direction perpendicular to the surface thereof by means of a sputtering process to form a magnetic layer (see, for example, Japanese Provisional Patent Publication No. 122232/1980).
One example of the perpendicular magnetic recording media will be described in reference to FIG. 2 of accompanying drawings. On at least one side of a substrate 1 comprising a heat-resistant polymeric film of a polyimide, a polyethylene terephthalate, an aromatic polyamide or the like, a magnetic layer 2 is formed which is composed of a Co-Cr system alloy layer or the like. Further, on this magnetic layer 2, a protective layer 3 and a lubricating layer 4 are provided in turn in order to complete the perpendicular recording medium. This type of perpendicular recording medium is applied to magnetic tapes and floppy disks.
The above-mentioned Co-Cr system alloy layer which is the magnetic layer may be formed on the substrate by a conventional sputtering process or a magnetron sputtering process in an argon atmosphere. The thus formed Co-Cr system alloy layer takes a column-like constitution oriented along the C-axis, and has a great saturation magnetization Ms which depends upon the composition of the Co-Cr system alloy layer, the high magnetic anisotropy energy Ku perpendicular to the surface of the layer and the large coercive force H.sub.C.sbsb..perp. , in a direction perpendicular to the surface of the layer, which magnetic properties are conditions for permitting the high-density recording.
However, in the case of forming the Co-Cr system alloy layer having desired magnetic properties in accordance with the above-mentioned producing method, it is required to heat the polymer film as the substrate at 80.degree. C. to 200.degree. C. in vacuum. Thus, a polyimide film and an aromatic polyamide film are preferred, since they are easier to prepare than a polyester film and have no problems such as deposition of oligomers on the film surface and the like.
However, in films such as the polyimide film and the aromatic polyamide film which are manufactured by a flow casting process and are required to be heated to remove a used solvent, there is a problem that some solvent still remains.
Concrete examples of the organic solvents include N,N'-dimethylacetamide, dimethyl sulfoxide, hexamethylphosphorylamide, N,N'-dimethylformamide, N-methyl-2-pyrrolidone, tetramethylurea, tetramethylenesulfone, phenol, phenol monohalide, cresol, cresol monohalide and xylene.
According to the above, when the polyimide film or the aromatic polyamide film having excellent heat resistance is heated in vacuum chamber as the substrate to prepare the ferromagnetic alloy layer, the solvent contained in the film will leak out on the surface of the substrate disadvantageously, with the result that the adhesive strength of the ferromagnetic alloy layer to the substrate will deteriorate. According to a running test on a track through a magnetic head, it has been found that the above-mentioned deterioration of the adhesive strength will facilitate the peeling of the ferromagnetic alloy layer from the substrate, which fact is considered to be fatal problem from the viewpoints of durability and reliability of the perpendicular magnetic recording medium.
The problem described above cannot be solved by such a usual surface treatment as a heat degassing treatment which may be carried out as a pretreatment for a sputtering process or vacuum evaporation.
For example, Japanese Provisional Patent Publication No. 129956/1984 discloses that an acrylic resin substrate for a magneto-optical recording medium is thermally treated under 10.sup.-4 Torr or less at 70.degree. to 85.degree. C. for 1 hour or more in order to degas the surface of the acrylic resin substrate and to thereby improve magnetic properties of the magnetic layer which will be formed on the substrate. However, the magneto-optical recording medium is used without contacting with the optical head, whereby the above-mentioned perpendicular magnetic recording medium is used in contact with the magnetic head. Thus in the case of this magneto-optical recording medium, the adhesive strength between the substrate and the magnetic layer is not as important.
On the other hand, when the residual solvent in the polyimide film is mainly N,N'-dimethylacetamide (boiling point: 165.degree. C. at 760 mmHg, 63.degree. C. at 12 mmHg), the amount of the residual solvent in the whole film will not change, even if the degassing treatment is carried out under 10.sup.-4 Torr or less at 100.degree. C. for about 1 hour. For this reason, the adhesive strength of the ferromagnetic alloy layer will not be improved and circular peeled portions between the substrate film and the magnetic layer will not be reduced in. A peeling phenomenon will occur during operation of the magnetic head probably due to the residual solvent in the film. The reason why the solvent is not removed from the film would be that the residual solvent interacts with molecules of the polymer and thus is difficult to vaporize from the film. In fact, in the case of the above-mentioned solvents, it has been shown from test results using a thermogravimetry device in the atmosphere that a weight change of the sample film due to the decrease of the residual solvent cannot be observed at a temperature less than 260.degree. C.
Further, it is important to reduce the amount of the residual solvent not only on the surface of the film but also in the whole film, because the solvent in the film will bring about, in addition to the formation of the circular peeled portions during running caused by the magnetic head, the deterioration of adhesive strength owing to thermal diffusion of the residual solvent from the interior of the film to a boundary surface between the film and the ferromagnetic alloy layer.